Prevalence of antibiotic resistance genes in soils after continually applied with different manure for 30 years.

A 30year field experiment with a wheat-soybean cropping system were performed to compare the long-term anthropogenic influence on soil ARGs. Compared with chemical fertilization, the occurrence of 38 ARGs and the abundance of seven ARGs (tetL, tetB(P), tetO, tetW, sul1, ermB, and ermF) were significantly increased by long term exposure of pig manure. However, application of wheat straw and cow manure not substantially affected the abundance of ARGs except cow manure increased the abundance of tetM and tetW to a detectable level. Relative abundance of Firmicutes, Gammaproteobacteria and Bacteroidetes and ARGs observed in the soil were significantly correlated. Integrase gene I1 (intI1) is commonly linked to genes conferring resistance to antibiotics, it was significantly increased in pig manure treated soils and showed a high positive correlation with the abundance of ARGs which were significantly affected by pig manure. Concentrations of Cu, Zn and Pb were also increased in manure treated soil and positive correlated with the relative abundance of intI1 and most of the ARGs. These results indicated that long-term animal manure application to soils has polluted the soil, especially for pig manure, and it should be scrutinized as part of future stewardship programs.